Tabular real column width

Question

I'm trying to put a table in the header of a document. In this document, width of the header is larger than the text body. The header needs to be exactly 183 mm wide and, in it, there is a table 183 mm wide with the width of the columns of 70, 93 and 20 mm. So I set the document as this

\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyheadoffset[L]{0.9cm} %header offset
\fancyheadoffset[R]{0.9cm} %header offset

whith those settings (and other that I've omitted) the width of the header is 183 mm (correct!)

Then I try to put the table in the header

\fancyhead[C]{
\begin{tabular}{|p{70mm}|p{93mm}|p{20mm}|}
\hline
first & second & third\\
\hline
\end{tabular}
}

This results in a table whose cells are wider than the value that I've used (something like 4 mm wider) and I have an overfull warning. For example, only to be clear, the first column is 74 mm wide rather than 70. I think that the difference is due to the fact that with p{70mm} I set the text width and not the column width and this give me a small offset on both margins (in fact there is a little space between the column line and the first letter).

Is there a way to set the column width as the distance between the two lines? Otherwise, I need to decrease the width of the columns of a few millimeters resulting in a less precise work.

Thanks!

Answer 1

You need to subtract 2\tabcolsep from each column like this:

\begin{tabular}{|p{\dimexpr70mm-2\tabcolsep}|p{\dimexpr93mm-2\tabcolsep}|p{\dimexpr20mm-2\tabcolsep}|}
\hline
first & second & third\\
\hline
\end{tabular}

But this method will leave some space before and after the first and last columns, respectively. If you want to remove that space use this column specification:

{|@{}p{\dimexpr70mm-\tabcolsep}|p{\dimexpr93mm-2\tabcolsep}|p{\dimexpr20mm-\tabcolsep}@{}|}